The nucleotide sequence of the end of trpB and the first 300 nucleotide pairs of trpA of E. coli have been determined by sequence analysis with mRNA labeled in vivo and in vitro. The comparable region of the trp operon of Salmonella typhimurium will now be sequenced, in order to obtain comparative information of base pair changes in DNA relative to amino acid changes in corresponding proteins. Intergeneric-intragenic recombinants generated in crosses between E. coli and S. typhimurium will be characterized in an attempt to determine the functional significance of the amino acid differences which have accumulated in the corresponding tryptophan synthetase Alpha chains of the two species. Four recombinant Alpha chains have been thoroughly characterized and new recombinant types will be isolated and studied. Polarity suppressors isolated in our lab and elsewhere are predominantly mutants in the structural gene for the rho termination protein. We are presently searching for the sites in the trp operon at which rho causes termination of transcription. When these sites have been located we will compare mutant and normal rho action, in an in vitro transcription system. BIBLIOGRAPHIC REFERENCES: C. Squires, F. Lee, K. Bertrand, C.L. Squires, M. Bronson & C. Yanofsky. Nucleotide sequence of the 5 feet end of tryptophan messenger RNA of Escherichia coli. J. Mol. Biol., in press. F. Lee, C.L. Squires, C. Squires & C. Yanofsky. Termination of transcription in vitro in the E. coli tryptophan operon leader region. J. Mol. Biol., in press.